Polarization is defined as the orientation of the oscillation of the electric field in the plane perpendicular to the electromagnetic wave's direction of travel. The electric field vector can be arbitrarily divided into two perpendicular components. The two components can have distinct amplitudes and phases. When the two components are in phase (i.e., where the phase difference is equal to 0 or 180 degrees), the electric field vector has a constant direction, the tip of the electric field vector traces out a single line; and this case is, therefore, called linear polarization. When the two components have the same amplitude and are exactly 90 or 270 degrees out of phase, the electric field vector traces out a circle in the plane, so this case is referred to as circular polarization. If the two components are out of phase by a factor not equal to 0, 90, 180, or 270 degrees, or if the two components do not have the same amplitude, the tip of the electric field vector traces out an ellipse; accordingly, this case is recognized as elliptical polarization.
Producing a light source that has a desired polarization is important for many applications. In satellite communication, two orthogonal polarization states (e.g., vertical and horizontal polarizations, or right- and left-circular polarizations) are used to achieve a two-to-one frequency reuse, thereby doubling the services a satellite can provide. Additionally, polarization is of great importance in chemistry and biomedical sciences due to the circular dichroism (CD) that is exhibited by many chiral molecules, such as various organic molecules, proteins and DNAs. Further, CD spectroscopy is based on the differential absorption of left- and right-circularly polarized light in specimens. Mechanical engineers, as well, use polarized light to detect strain and stress in prototypes because strain and stress induces birefringence. Laser sources with a variety of polarization states are also used for quantum cryptography.
Many light sources [e.g., light-emitting diodes (LEDs)], however, are randomly polarized. Semiconductor lasers are mostly linearly polarized—either transverse electric (“TE”—i.e., wherein the electric field lies in the plane of the device material layers) or transverse magnetic (“TM”—i.e., wherein the electric field is perpendicular to the plane of the device material layers); the polarization is determined by the optical selection rules of the gain medium. Intrinsically polarization-controllable light sources are highly desirable but are very challenging to build. For example, the realization of circularly polarized lasers or LEDs has proven elusive.
Conventionally, selection and manipulation of the polarization state of light output is conducted externally using bulky and usually expensive optical components. To create linearly polarized light, absorptive polarizers (e.g., wire-grid polarizers), or beam-splitting polarizers (e.g., Nicol prisms and Wollaston prisms) are used. To change the polarization direction of linearly polarized light, absorptive polarizers and half-wave plates are used. To generate circularly and elliptically polarized light, wave plates are used. The latter are based on birefringence and work by producing phase shift between the two perpendicular components of the electric field of a beam of light. Many of the optical components require customer design and manufacturing, and some of the optical components are only available for a limited range of spectrum. For example, wave plates are usually available from the visible to near-infrared spectrum regimes (λo=450˜1600 nm).